J. Phys. Chem. B 2001, 105, 11447-11455
11447
Self-Assembly of Bolaamphiphiles Forming Alternating Layer Arrangements with Lead and Copper Divalent Ions R. Buller,† Hagai Cohen,† T. R. Jensen,‡ K. Kjaer,‡ M. Lahav,*,† and L. Leiserowitz*,† Materials and Interfaces Department, Weizmann Institute of Science, RehoVot 76100, Israel and Condensed Matter Physics and Chemistry Department, Risø National Laboratory, DK 4000 Roskilde ReceiVed: May 1, 2001; In Final Form: September 4, 2001
Five bifunctional R-amino acid--carboxy bolaamphiphiles [(2-R,S, or RS)-(HOOC)-(CH2)m-CONH-(CH2)nCH(COOH)(NH2) m)20,22, n)3,4 labeled (l), (d,l)-C22Orn (l), (d), (d,l)-C22Lys, (d,l)-C24Lys respectively] were synthesized. These molecules were deposited on different aqueous subphases, and studied by means of grazing incidence X-ray diffraction (GIXD). On deionized water, the bolaamphiphiles (d,l)-C22Lys yield a mixture of crystallites: both a monolayer, in which the chains are tilted from the normal, and a multilayer in which the molecules lie parallel to the water. On the other hand, when deposited on mono- or bi-metal ionic subphases, they self-assemble into crystalline multilayer films in which the molecules lie parallel to the aqueous solution surface, linked head-to-head and tail-to-tail in the form of extended chains. The latter are juxtaposed such that the metal ions form sheets separated by the organic molecules, aligned perpendicular to the plane of the aqueous solution. Deposition of either the enantiomerically pure or racemic bifunctional bolaamphiphiles on an aqueous solution of mixed Cu(Acetate)2 and Pb(Acetate)2, yields self-assembled crystalline films composed of the two different metal cations, arranged in alternating sheets, separated by the organic spacer. Both GIXD and X-ray-photoelectric-spectroscopy (XPS) studies (after deposition on solid support) demonstrate that the structures of these films differ form the ones formed on either pure Pb(Ac)2 or on Cu(Ac)2 solutions, thus excluding the simultaneous formation of the two monometallic crystalline phases.
Introduction The construction of functional materials via a step by step process from the molecular level, utilizing supramolecular architectures, is attracting increasing interest.1 Among these systems are ordered hybrid organic/inorganic composite materials, in particular quantum size nanoparticles orderly embedded within rodlike molecular matrixes, which display cooperative optoelectronic properties, and receive special attention.2,34 Recently, we demonstrated that self-assembled organometallic architectures can be generated by depositing the organic ligand on aqueous subphases containing a metal cation of a single type. The resulting films formed on the aqueous solution, contain ions organized in the form of periodic sheets separated by alkane-dicarboxylic acid molecules, which are aligned parallel to the solution surface,3 or in oriented silver ion grids of (2 × 2) or (3 × 3) in the form of mono- or bi-layers.4,5 The structures of the crystalline films were determined at the sub nanometer scale. Such architectures are appropriate starting materials for the preparation, by topotactic reaction, of hybrid organic/inorganic composites of nanoparticle crystallites separated by organic spacers arranged in periodic lattices.6,7 We are currently interested in preparing hybrid systems comprising two types of nanocrystallites of metal or semiconductor character, arranged in an alternating manner, and separated by organic spacers. Here, we describe the preparation and characterization of an ordered three-component film, prepared by self-assembly, at the air-solution interface. This * To whom correspondence should be addressed. † Materials and Interfaces Department, Weizmann Institute of Science. ‡ Condensed Matter Physics and Chemistry Department, Risø National Laboratory.
film is composed of two different metal cations, arranged in alternating sheets, separated by an organic spacer, as shown in Scheme 1a,b. These metal sheets will serve as precursors for the eventual formation of the nanocrystallites. As the organic matrix, we used a nonsymmetric bolaamphiphile, composed of two different functional headgroups where each selectively binds to a different ion, extracted from a mixture of two ions in the subphase. As a model system, we made use of the “bolaamphiphilic series” (Scheme 2 Vi, Vii, Viii) including (2-R), (2-S) or (2-R,S)-2-amino-7-aza-8-oxo-nonacosane-1,29-dicarboxylic acid (HOOC)-C20H40-CONH-C4H8CH(COOH)(NH2) [labeled d, l or (d,l)-C22Lys], as well as l or (d,l)-C22Orn and (d,l)-C24Lys. These molecules contain carboxylic acid and R-amino acid headgroups at opposite ends of the chain. It was anticipated that when spread on a solution containing a mixture of Cu2+ and Pb2+ metal ions, the former will bind exclusively to the amino acid groups, and the latter to the carboxyl group only, forming a head-to-head and tailto-tail arrangement of extended chains. Although there is a marked difference of 7 orders of magnitude in the dissociation constants (Kd) of amino acids with each of the two metal ions (pKd ≈ 15 for Cu2+ and pKd ≈ 8 for Pb2+),8,9 only a difference of 0.3 in dissociation constant exists for the complexes of carboxylic acid with these two ions (pKd ≈ 4 for Pb2+ and pKd ≈ 3.7 for Cu2+).10,11 In light of these differences, it was logical to deduce that Cu2+ will bind exclusively to the amino acid headgroup, but there was an ambiguity whether Pb2+ would bind selectively to the carboxylic acid. An indication of selective binding resulted from an analysis of the metal content of a three-component crystal containing Cu2+, Pb2+ and (d,l)-glutamic acid. Atomic absorption experi-
10.1021/jp0116508 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/31/2001
11448 J. Phys. Chem. B, Vol. 105, No. 46, 2001 SCHEME 1: Schematic Representation of Two Possible Packing Arrangements of Bolaamphiphile Molecules Connecting Two Different Metal Ion Types Arranged in Alternating Sheets: (a) View Perpendicular to the Solution Surface, Where the Arrow in the Hydrocarbon Chain Represents an Amide Moiety, (b) View of the Arrangement Edge on the Air-solution Interface. (c) Undesired structure where neither of the metal ionic sheets is of a single type
Buller et al. SCHEME 2: Synthetic Route for the Preparation of the Bolaamphiphilic Series
ments show that the crystal contains a 1:1 ratio between copper and lead. The details are described in the Experimental Section. The role of the amide group located adjacent to one of the polar headgroups in the hydrocarbon chain is to promote, by virtue of the N-H‚‚‚O hydrogen bond, translation symmetry between molecules along the a axis (Scheme 1a), as opposed to the undesired structural motif in Scheme 1c, and thus yield formation of alternating sheets of Pb2+ and Cu2+ ions. Results a. Synthesis. The bolaamphiphiles designed (Scheme 2Viiii), are produced by regioselectively condensing the amine of lysine or ornithine methyl esters, with R,ω-dicarboxylic acid monomethyl ester I, similar to the use of isobutyl chloroformate as an activating group.12,13 The ester protection groups are removed by hydrolysis, yielding compound V. The overall process is depicted in Scheme 2. b. Isotherms. Preliminary evidence for binding of the divalent copper and lead ions from the aqueous subphase to the bolaamphiphile at the solution surface was obtained from the surface pressure-molecular area (π-A) isotherms (Figure 1).
The isotherm for the film on the mixed ionic solution is very different from that of the film on pure water. It also differs from that of the film formed when spreading the bolaamphiphile on pure Cu(Ac)2 and Pb(Ac)2 subphases. Furthermore, the relatively low limiting areas per molecule of the films on the different subphases are consistent with multilayer formation.
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TABLE 1: Analysis of the Separated Peaks
subphase
literature (eV)
N(1s)
BE
amide amine ammonium
399.6 400.0 401.3
a deionized water (eV) BE
b Pb(Ac)2 mM (eV)
c Cu(Ac)2 1 mM (eV)
d Cu(Ac)2 & Pb(Ac)2 1 mM (eV)
%
BE
%
BE
%
BE
%
399.7
50
50 50
399.4 400.0
50 50
50
53 15 32
399.6 400.2
401.5(0.1)
399.6 400.2 401.4
a
Relative errors in ratios are 5%, errors in peak positions are 0.03 eV unless otherwise stated. Bottom: Experimental spectra and curve fitting to the N(1s) lines.
Figure 1. Surface pressure-area (π-A) isotherms of bolaamphiphile d-C22Lys, on: (1) deionized water, (2) Pb(Ac)2, (3) Cu(Ac)2, and (4) mixed Cu(Ac)2 and Pb(Ac)2 solution as subphase.
c. X-ray Photoelectron Spectroscopy. Supported multilayers of d or (d,l)-C22Lys with or without metals were prepared by spreading the bolaamphiphile, on each of the four roomtemperature subphases: deionized water, solution containing Pb(Ac)2, Cu(Ac)2, and a 1:1 mixture of Pb(Ac)2 and Cu(Ac)2. These films were then transferred onto a clean glass support by slow draining deposition.14 The glass with the deposited film prepared on the metal containing subphase, was then washed with deionized water to remove excess acetate and metal ions, and then dried. X-ray photoelectron spectroscopy (XPS) measurements on the mixed film demonstrate the 1:1 ratio of the two metal types, as well as the 1:2 ratio between each metal ion and the bolaamphiphile d-C22Lys. The relative atomic concentrations of d-C22Lys spread on a 1:1 solution of mixed Pb(Ac)2 and Cu(Ac)2 and transferred onto a glass support are: Cu:Pb:N:C 1:1:4:56 based on calculation, and 1:1:4:59 as found experimentally. Relative errors in ratio of 3% are found for both the Pb(4f) peak binding energy (BE) ) 139.0(0.15) eV and the C(1s) peak BE ) 248.8(0.05) eV; and 10% for both the Cu(2p3/ 2) peak BE ) 934.7(0.20) eV15 and the N(1s). The concentration of oxygen O(1s) could not be unambiguously determined because of the overlap by the oxygen signal originating from the glass support. We note that copper undergoes fast reduction under the experimental conditions.16-19 The C(1s) broad peak around 288.3 eV (Figure 2) originates from the carboxylate anion (-COO-) line at 288.5 eV, the amide group’s ketone (-CONH-) at 288.0 eV and the carboxylic acid (-COOH) at 289.0 eV when present.20 A lower intensity at ∼289.4 eV is observed in curves (c) and (d) (Figure 2), indicating an induction in ionic character of the carboxylate group, -COOH f -COO-. This change occurs only in the presence of Pb2+ ions, which indicates binding thereof to the carboxylate group. It should be noted that residual carboxylate from the unwashed acetate may be present in these samples. Yet, its quantity is estimated to be less than 15% of the carboxylate signal. Evidence for the preferential binding of Cu2+ to the R-amino acid ligands arises from the XPS signals of films of d or (d,l)-
Figure 2. Carboxyl binding energy of d-C22Lys on different subphases. (a) pure Cu(Ac)2, (b) deionized water, (c) pure Pb(Ac)2, (d) mixed Cu(Ac)2 and Pb(Ac)2. Arrows indicate the carboxyl and carboxylate binding energies. The main aliphatic C(1s) peak is at 284.8 eV.
C22Lys spread on the different subphases (Table 1, Figure 3). Two signals are resolved in the N(1s) line,20,21 the first at 399.6 eV is assigned to amide -CONH-, whereas the second appears at 400.0 eV for the amine -CH(COO-)(NH2) or at 401.3 eV for the ammonium -CH(COO-)(NH3+). The nitrogen atom serves as a clear indicator of metal binding because its signal originates only from the bolaamphiphile. Pure amino acids have a tendency to crystallize as zwitterionic pairs. According to the XPS spectra, this motif is evident both for the pure bolaamphiphiles deposited on deionized water (see peak position at 288.5 eV in Figure 2b, and Figure 3a), and on the solution containing pure Pb(Ac)2 (see peak position at 288.5 eV in Figure 2c, and Figure 3b). The presence of the weak amine (NH2) peak in Figure 3b serves as an indication of the binding of lead to the amino acid functional group, most probably due to the high value of pKd ≈ 8 of lead amino acid complex.8,9 The difference in binding energy of 1.3 eV, corresponds to the change in ionic character of the amino acid nitrogen (NH3+ f NH2), and is a clear indication of copper amino acid complexation. This difference in binding energy was observed both for the bolaamphiphile on Cu(Ac)2 subphase (Figure 3c), and on the mixed Cu(Ac)2 and Pb(Ac)2 subphase (Figure 3d). Despite the large overlap between the two peaks (in Figure 3c, and in 3d) there is no doubt that they correspond both to the amide and amine, and thus exclude any possible presence of the ammonium ion. We may conclude from these data that the divalent copper and lead ions, each bind to its appropriate ligand, as depicted in Scheme 3, thus indicating a head-to-head and tail-to-tail arrangement (Scheme 1a,b). d. Grazing-Incidence X-ray Diffraction. The anticipated structure of the metal complex (Scheme 1a,b), was verified via an analysis of the grazing incidence X-ray diffraction (GIXD) measurements performed on the various films, in situ, at the air-solution interface. The GIXD experiments22,23 were per-
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Figure 4. Surface plot of the scattered intensity I(qxy, qz) showing the {0,k} reflections of (d,l)-C22Lys spread on different subphases: (a) mixed Cu(Ac)2 and Pb(Ac)2, (b) pure Pb(Ac)2, and (c) pure Cu(Acetate)2.
Figure 3. XPS N1s energy measurements of bolaamphiphile d-C22Lys spread on different subphases: (a) Deionized water, (b) Pb(Ac)2 solution, (c) Cu(Ac)2 solution, and (d) mixed Cu(Ac)2 and Pb(Ac)2 solution as subphase.
SCHEME 3: Schematic Representation of the Repeating Unit of the Bolaamphiphiles on Different Subphases
formed on the liquid surface diffractometer, at the synchrotron beamline BW1, HASYLAB. A brief description of the method is given in the Experimental Section. Here, we shall describe
first the diffraction spectra obtained from the bimetallic line array, and then the data obtained from the non metallic species. d.1. Three-Component System. Deposition of the bolaamphiphiles on pure Pb(Ac)2 and on Cu(Ac)2 solutions, yields crystalline films with d-spacings of 76 Å and 71 Å respectively24 (see figures 4, 5). Making use of the mixed Cu-Pb subphase resulted in the self-assembly of a different crystalline film, that incorporates both metallic ions. This difference is manifest from a comparison of the GIXD patterns of d or (d, l)-C22Lys on the mixed solution of Cu(Ac)2 and Pb(Ac)2, with those of the bolaamphiphile on the pure Cu(Ac)2 and Pb(Ac)2 subphases. The GIXD pattern of the mixed Cu-Pb system (Figure 5a,b) cannot be formed by a superposition of the Bragg peaks of the Cu and the Pb systems, since they lie at different qxy positions. This analysis thus excludes the simultaneous formation of two monometallic crystalline phases, in the mixed Cu-Pb case. The X-ray scattered intensity, I(qxy,qz), of the bolaamphiphile d-C22Lys on Cu(Ac)2 and Pb(Ac)2 mixed subphase, is presented in Figure 6b in the form of a 2D surface contour plot. The presence of Cu2+ and Pb2+ ions with very different X-ray scattering amplitudes, arranged in successive sheets in the crystalline film, facilitates the assignment of the chain axis in the series. The X-ray diffraction peaks observed at low Bragg angles (Figure 6b) could all be assigned Miller indices {0,k}, yielding a d(01) lattice spacing between 70 and 80 Å, depending upon the length of the molecules in the series, indicating that the chain axes lie parallel to the solution surface. The strong dependence of d(01), on the molecular length, is consistent with a structure whose chain axes lies parallel to the solution surface (see Table 2). This follows from the differences in the observed (01) lattice spacing (Åo), which correspond nicely to the differences between the calculated lengths (Åc) of the molecules in the series [dl-C22lys-dl-C22Orn) 3.0 Åo, 2.4 Åc, ddl-C22lysddl-C22Orn ) 3.1 Åo, 2.4 Åc, ddl-C24lys-ddl-C22Lys ) 5.2 Åo, 4.8 Åc]. The d(01) spacing corresponds to the length of two
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J. Phys. Chem. B, Vol. 105, No. 46, 2001 11451
Figure 5. (a) The positions (along qxy) and intensities of the Bragg peaks, integrated along qxy and qz measured for d-C22Lys amphiphile spread on different subphase solutions:mixed Cu(Ac)2 and Pb(Ac)2, Cu(Ac)2, Pb(Ac)2 (b) Corresponding results for (d,l)-C22Lys amphiphile spread on the three different subphases.
bolaamphiphilic molecules whose chain axis lies parallel to the solution surface. These molecules are linked head-to-head and tail-to-tail, via two different metal ionic moieties with the appropriate metals, as shown in Scheme 1a,b. There are no significant differences in the GIXD patterns of the enantiomerically pure and racemic molecules, probably because both chiral and racemic amino acids tend to form planar copper complexes with similar geometry. Both bolaamphiphiles (chiral and racemic) form crystallites with similar d(01) spacings, however, the peaks differ in their relative intensities, which suggest somewhat different structures. This result also precludes the possibility of spontaneous resolution of the racemate. The difference in the structure is manifested in the symmetry element linking two copper-bound amino acid groups, centered about the metal ion, which is a 2-fold axis for the enantiomer and an inversion center for the racemate. The experimental shape of the Bragg rods, that peak at qz ) 0 Å-1, implies an arrangement where the bound metal ions are aligned in sheets perpendicular to the solution surface. The full width at half-maxima (fwhm) of these Bragg rods along qz, provide an estimate of the film thickness Lz ≈ 40 ( 10 Å, according to the formula Lz ≈ 0.88*2π/[fwhm] (see Experimental Section), which would correspond to about seven layers. It is possible to estimate the angle γ formed between the molecular chain axis and the plane of the metal sheet by comparing the observed d spacing (between the metal sheets) with the estimated length L of the molecular chain axis. The angle γ is given by sinγ ) d/L, according to which γ is, on average, 74° (see Table 2). A more quantitative analysis of the GIXD data was carried out for homochiral d-C22Lys on mixed Cu-Pb subphase. The symmetry element linking two chiral molecules via the metal ions can only be a 2-fold axis. This symmetry axis may in principle be either perpendicular or parallel to the solution
Figure 6. d-C22Lys on a mixed Cu(Ac)2 Pb(Ac)2 subphase solution. (a) Observed (dots) and calculated (line) Bragg rods I(qz) for a model composed of six layers. (b) Low angle 2D surface plot of the scattered intensity I(qxy, qz) showing the {0,k} reflections.
TABLE 2: Long Axial Spacing of the Bolaamphiphilic Series on Mixed Cu(Ac)2 and Pb(Ac)2 Subphase compound
measured d spacing a (Å)
calculated axial length (Å)
deduced γ angle (°)
l-C22Orn (d, l)-C22Orn l-C22Lys d-C22Lys (d, l)-C22Lys (d, l)-C24Lys
70.0 70.2 73.0 73.4 73.3 78.5
73.6 73.6 76.0 76.0 76.0 80.8
72 73 74 74 75 76
a These values lie within an error range of 0.5 Å, which is acceptable regarding the broad Bragg peaks and experimental error.
surface. The former possibility is more probable, in view of the deduction from the Bragg rods, that the metal sheets are aligned perpendicular to the aqueous surface (vide supra). Furthermore, the calculated angle of γ ≈ 74° between the molecular chain axis and the metal sheets precludes a 2-fold axis parallel to the solution surface. Both structures were subject to modeling using Bragg rod profiles for a unit cell of plane symmetry p2. Making use of the SHELX97 computer program,25,26 a reasonable fit to the observed Bragg rod profiles was obtained for a unit cell on which the 2-fold axis was aligned perpendicular to the water plane. The Cu2+ and Pb2+ ions were centered on the 2-fold axis, and the bolaamphiphiles constrained as rigid rods (Figure 6a). The model structure best fitted to the experimental data, suggested that the molecules lie in a plane parallel to the solution surface, and their long axes making an angle of γ ) 79° from the plane of the metal sheets (see Scheme
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Figure 8. (d,l)-C22Lys on pure water. GIXD intensities plotted as a surface plot against qz and qxy
Figure 7. (a) 2D surface plot of the scattered intensity I(qxy, qz) showing reflections of (d,l)-C22Lys spread on mixed Cu(Ac)2 and Pb(Ac)2 subphase. b) The positions (along θxy) and intensities of the Bragg peaks, integrated along qz (between 0 Å-1 and 0.15 Å-1) measured for amphiphile series spread on mixed Cu(Ac)2 and Pb(Ac)2 subphase.
TABLE 3: Reflections Arising from the Interchain Arrangement d (Å) 4.6 4.2 3.8
weak, sometimes present strong, always present weak, always present
qxy
qz
1.40 1.52 1.67
0< qz
